In the manufacture of semiconductor memory devices, it is known that production yield can be increased by provision of a set of redundant cells. With the availability of redundant cells, a certain number of defective cells in a device become tolerable because they can be substituted by the redundant cells. The increased tolerance allows production yield to increase.
In a typical manufacturing process of memory devices with redundant cells, a test is performed after a memory device is fabricated to determine if it contains any defective cells. If a defective cell is found, the entire row or column of cells containing the defective cell is substituted. The substitution is achieved by programming the row or column address into an internal programming circuit. During normal operation of the memory, if the address of an memory access is the same as the content of the programming circuit, the access is directed to the redundant cells.
The programming circuit typically comprises a set of fuse elements, each for programming an address bit. The number of fuse elements provided in the programming circuit depends upon the substitution scheme. For example, if the substitution is performed on a row basis (i.e. the entire row of cells is substituted when one cell therein is found to be defective) , then the number of fuse elements required may be equal to the width of the row address. On the other hand, if the substitution is performed on a column basis (i.e. the entire column of cells is substituted when one cell therein is found to be defective), then the number of fuse elements required may be equal to the width of the column address.
In most prior art fuse programming circuits, each fuse element is connected in series with a resistor between a first voltage source (e.g. Vdd) and a second voltage source (e.g. Vss) as shown in FIG. 1. The programming circuit is programmed by selectively removing the fuse elements based upon the row or column address of the defective cell. The fuse can be removed either by laser-cutting or by passing a sufficiently high current through the fuse. A specific example of the later technique is disclosed in U.S. Pat. No. 4,532,607 issued on Jul. 30, 1985 to Y. Uchida.
When the fuse is removed, the output voltage of the circuit is equal to Vdd. When the fuse element is kept, the output voltage is equal to Vss. The output voltage level represents the binary value of the corresponding bit in forming a defective cell's address.
One problem with the above described prior art fuse programming circuit is that when the fuse element is preserved, a constant current flows through the resistor from Vdd to Vss,, causing power consumption and heat dissipation. To reduce such power consumption and heat dissipation, the value of the resistor can be increased. Unfortunately, the resistance required is typically in the order of megohms. To provide a resistance of such magnitude requires a high-resistive mask layer and such a requirement is undesirable in a semiconductor manufacturing process.